Utah State University DigitalCommons@USU All U.S. Government Documents (Utah Regional U.S. Government Documents (Utah Regional Depository) Depository) 1978 Quaternary Rhyolite from the Mineral Mountains, Utah, U.S.A. S. H. Evans, Jr. W. P. Nash University of Utah, Department of Geology and Geophysics Follow this and additional works at: https://digitalcommons.usu.edu/govdocs Part of the Geology Commons Recommended Citation Evans, Jr., S. H.; Nash, W. P.; and University of Utah, Department of Geology and Geophysics, "Quaternary Rhyolite from the Mineral Mountains, Utah, U.S.A." (1978). All U.S. Government Documents (Utah Regional Depository). Paper 597. https://digitalcommons.usu.edu/govdocs/597 This Report is brought to you for free and open access by the U.S. Government Documents (Utah Regional Depository) at DigitalCommons@USU. It has been accepted for inclusion in All U.S. Government Documents (Utah Regional Depository) by an authorized administrator of DigitalCommons@USU. For more information, please contact [email protected]. DEPARTMENT OF I GEOLOGY AND GEOPHYSICS TITLE: Quaternary Rhyolite from the Mineral Mountains, Utah, U.S.A. AUTHORS: s. H.-Evans, Jr. and W. P. Nash DATE: March 1978 UNIVERSITY OF UTAH SALT DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency Thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. 3 P Quaternary Rhyol ite from the Mineral Mountains, Utah, U.S.A. P FINAL REPORT: VOLUME 77-10 DOE/DGE EY-76-S-07-1601 S. H. Evans, Jr. and W. P. Nash ______. .___ ~. DISCLAIMER Nrlther the Unlted Slates Government no1 any agency thereof. nor any 01 ?hell emuloyeei. #makermy warranty, express or Imglled. or aliumer any legal l#ab#I#iyor ieraonr#b#l~rylor the dCCurdc~. cornoletenerr. or uretulneir of any 8niormatmn apparaws product. or P~OCCII diiclared or represent5 that 81s use Wuld not mfrtnge pr8vately owned r8Qhtr Reference here," 10 any Ipec#l#c comrneic~al Druduct. procesi. or mrvice by trade name. trademark. manufacturer or ofherwise. does not necessarily ~onmt~teor 8mply 11s endoriemem recommendation. Or favoring by fflr United Stales Government or any agency thereof The views and o~inioniof aulhori expressed here," do not necesmr~lystate or reflect lhore of tne Un~teCSralei Governmen1 or any agency thereat TABLE OF CONTENTS Page Abstract.. ........................ 1 Introduction ........................ 3 General Geology ...................... 3 Volcanic Rocks ....................... 7 Petrography ........................ 10 Mi neral ogy ......................... 13 Crystallization of Feldspar ................ 24 Chemistry ......................... 36 D Geothermometry ....................... 46 Water Fugacity ....................... 50 Petrogenesis ........................ 51 Acknowledgements ...................... 55 References .......................... 56 1 Ga a ABSTRACT d A suite of s icic volcan,c rocks is associatec, wit,, the Roosevelt Hot Springs geothermal area in southwestern Utah. The volcanic sequence b includes Tertiary rhyolite 8 m.y. old and obsidian, ash and rhyolite of Quaternary age. The Quaternary lavas are characterized by hiqh silica content (76.5% Si021 and total alkalies in excess of 9 percent. Obsidians commonly contain greater amounts of fluorine than water. Two older flows (0.8 m.y, 1 can be distinguished from younger dome and pyroclastic material Y (approximately 0.5 m.y. ) by subtle differences in their chemistry. The mineralogy of the rhyolites consists of alkali feldspar, plagioclase , and small amounts of Fe-Ti oxi des , biotite, hornblende and rare allanite. Fe-Ti oxide temperatures are 740-785OC for the flows and 635-665% for the domes; two feldspar temperatures give similar results. The phase re1ationshi ps of bulk rock, glass and feldspar compositions demonstrate that the younger Quaternary rhyolites could have been derived from the earlier magma type, represented by the obsidian flows, by a process of crystal fractionation. The major phases which must fractionate are a1 kal i feldspar, plagioclase and quartz wf th minor amounts of biotite, magnetite and ilmenite participating also. Q Trace element patterns support this scheme as well. The Tertiary b 2 0 1 lavas cannot be related to the Quaternary rhyolites and are thought to represent a separate event. P P 3 .Introduction ---~ Young silicic volcanics have been considered to be indicators of the presence of shallow magma chambers with sufficient heat remaining to serve as possible geothermal resource centers. In the Mineral Mountains of western Utah obsidian flows and rhyolite domes ranging in age from 0.8 to 0.5 m.y. occur discontinuously for about 15 km. along the range crest. Previous studies of this area (Earl1 1957; Liese, 1957) interpreted all rhyolites as the remnants of a large silicic volcano of Pliocene age. Evidence presented here and in Lipman et al. (1978) demonstrates that many separate lava flows, ashes and domes were erupted along the range crest. Along one of the western range front faults, about 2 km northwest of the nearest rhyolitic volcanics, Roosevel t Hot Springs i s 1 ocated within the Known Geothermal Resource Area (K.G.R.A.), which has been proven to be a commercial area. The ii si 1 i ci c vol cani sm recorded by the rhyol i tes of the Mi neral Mountains perhaps provides the heat source for the K.G.R.A. (Lipman et al., 1978; Evans and Nash, 1975). -----General Geology The Mineral Mountains, located in west-central Utah (Fig. 11, near the eastern margin of the Basin and Range province, rise about 1 km above Escalante Valley to the west and an unnamed valley to the east. The range is nearly 50 km long and averges 10 km in width. 9 A The northern end of the range consists of a domal sedimentary sequence of early Paleozoic and late Mesozoic sediments. The western edge of 4 Fig. 1. Generalized geologic map of the central Mineral Mountains, Utah. 9 The sample localities are as described in Table 1. Hachured areas are Quaternary rhyolite domes, dark shaded areas are Quaternary ashes, light shaded areas are Quaternary rhyolite flows. Qal -a1 luvium, Qcal-cemented a1 luvium, Qos-opal and opaline sinter, Qb-basaltic cinder and flows, Trc-rhyolite domes in Corral Canyon, Tg-grant tic basement, P-Permian sediments, 9 pf2-Pre-Cambrian basement. A-Bailey Ridge, B-Wild Horse Canyon, C-Corral Canyon, D-Ranch Canyon, E-Bearskin Mountain, F-Little Bearskin Mountain, G-North Twin Flat Mountain, H-South Twin Flat Mountain. The geology is taken from a detailed geologic map published by the University of Utah (Evans, 1977). Y Y 3 5 4 P Y 38' 22' JON+ 112O 55' 112'45' 6 the range consists of numerous outcrops of rock, probably Precambrian in age, referred to as the Wildhorse Canyon Series by Condie (1960). The southern end of the range is a rotated fault block containing Paleozoic and Mesozoic sediments overlain by Tertiary intermediate and basic volcanics. Intruding these rocks is the Mineral Mountains pluton nearly 250 km2 in extent. This pluton is granitic in composition and contains many pegmatite and aplite dikes as well as mafic dikes of limited dimensions. Two potassium-argon ages of 15 and 9 m.y. (Park, 1968; Armstrong, 1970), indicate that the pluton may be i3 relatively young with respect to the majority of Basin and Range intrusive rocks. However, the possi bi1 ity exi sts that partia1 argon loss occurred during a volcanic episode in the area approximately 8 my. ago. Rb-Sr systematics suggest that the maximum age of the pluton is 35 m.y. (Lipman et al., 1978). Prior to eruption of rhyolitic volcanics of the Mineral Mountains the pluton and its adjacent country rocks were deeply eroded forming a rugged topography much like that of the present. North striking bounding faults mark the western and eastern margins of the Mineral Mountains. In the central portion of the Range the trace of a major fault on the west is marked by elongated mounds of siliceous sinter, u opal , and other hydrothermally a1 tered and cemented a1 1uvium. The abandoned Roosevel t Hot Springs resort, where water temperatures of 9OOC were recorded (Mundorff, 19701, is located on this major fault. 9 The chemical composition of water from a nearby seep yields a wall n rock equi 1 ibration temperature of 240°C using the Na-K-Ca thermometer 7 d of Fournier and Truesdell (1973). In this same area soil temperatures of 95 to 97OC have been measured at depths of 1 to 3 meters. The region is currently being developed for power production. Exploration drill holes and test wells have produced fluids with temperatures up to approximately 260OC at depths ranging from 300 to 1800 m. Volcanic Rocks 9 In the Mineral Mountains three distinct Quaternary volcanic rock types occur. Oldest are obsidian flows with intermediate aged ashes followed by younger rhyolite domes. A complete description of the occurrence and volcanic features is given by Lipman et al. (19781, and only a brief discussion is offered here. The obsidian flows are unique in their apparent fluidity. The flows, 3 occurring along Bailey Ridge and Wild Horse Canyon (Fig.